[vtkusers] Re: Rendering slowed down after recent CVS update

[Francois Bertel]

Hi Paul,

Please do a "cvs -q -z9 up -PdA", performance should be back for your card. Just let me know.

I modified VTK/Rendering/vtkOpenGLExtensionManager.cxx

Paul Melis wrote:
> Hello Francois,
> 
> Well, I upgraded the driver to 96.43.01. As I expected it doesn't make
> the OpenGL version reported pre-1.4. In fact, it now reports it supports
> OpenGL 1.5.8 instead of 1.5.6 :)
> See the attached glxinfo output.
> 
> Paul
> 
>> Paul Melis wrote:
>>> / I am very doubtful it will make a difference. In NVidia's OpenGL
>> />/ extension specifications it says that this card (based on an NV25
>> chip)
>> />/ does not support the extension. See
>> />/
>> http://developer.download.nvidia.com/opengl/specs/nvOpenGLspecs.pdf   An
>> />/ earlier version of this document (dated somewhere in 2003) clearly
>> marks
>> />/ the extension as being emulated in software for NV2x cards, with a
>> note
>> />/ to expect slow performance.
>> />/ I attached the output anyway.
>> /
>> Thanks for your feedback.
>>
>> Actually, I think it might make a difference :-) If the new driver
>> upgrade reports an OpenGL version less than 1.4 (currently it reports
>> OpenGL 1.5), and does not report "GL_EXT_blend_func_separate" in its
>> list of extensions, you will have your performance back without
>> changing any VTK code.
>> I'm explaining that later in this message.
>>
>> Anyway, I'll take a look at pages 6 to 10 of the nvidia document to
>> see if there is any other functionality implemented in software and
>> try to make the vtkOpenGLExtensionManager aware of that.
>>
>>> / Interestingly, the extension list doesn't
>> />/ have the blend_func_separate extension, while VTK obviously gets some
>> />/ function pointer when querying for this extension.
>> /
>>
>> The VTK behavior is the right one.
>>
>> Extension "GL_EXT_blend_func_separate" was promoted as a core feature
>> of OpenGL 1.4.
>> VTK first checks if this version of OpenGL is supported by the card,
>> if so, it uses the function pointer of "glBlendFuncSeparate".
>> If not, VTK gives a second chance by checking directly for the
>> extension. If this is supported, it uses the function pointer of
>> "glBlendFuncSeparateEXT".
>>
>> The Linux driver of your graphics cards says it supports OpenGL 1.5 so
>> the VTK logic falls in the first case.
>>
>> It has to happen in this order because some OpenGL implementations
>> only report the OpenGL version they support and they put only extra
>> extensions in the list of their extensions.
>>
>> For instance, on the mac, if you want to use 3D textures, the graphics
>> card says it supports OpenGL 1.2 (which has GL_EXT_texture3D as a core
>> promoted extension) but it does not list
>> GL_EXT_texture3D in its list of extensions.
>>
>> Ref: OpenGL 2.1 spec, Appendix G "Version 1.4", G.9 "Separate Blend
>> Functions", page 335 (pdf page 349).
>> http://www.opengl.org/registry/doc/glspec21.20061201.pdf
>>
>>
>> To sum up, please still give a try to a driver upgrade and let me know
>> about it.
>>
>> Regards.
>>
>>
>>>> / Paul Melis wrote:
>> />>>/ / Paul Melis wrote:
>> />>/ />/ />>/ Sean McBride wrote:
>> />>/ />>/
>> />>/ />>>/ /On 1/16/08 4:24 PM, Paul Melis said:
>> />>/ />>/ />/
>> />>/ />>/ />>/And another reply to myself...
>> />>/ />>/ />>/After going back to a CVS version of VTK of October 1st the
>> />>/ />>/ problem />>/seems to be gone. So something changed in VTK
>> between
>> />>/ />>/ october and today />>/that influences the render performance,
>> />>/ probably
>> />>/ />>/ due to a software />>/fallback suddenly being used in the
>> NVidia
>> />>/ driver.
>> />>/ />>/ />>/    />>/
>> />>/ />>/ />/
>> />>/ />>/ />/Next try Nov 1, Dec 1, etc. until you have it narrowed
>> down to
>> />>/ />>/ between 2
>> />>/ />>/ />/days... tedious, but easy.  You'll probably find the problem
>> />>/ this way.
>> />>/ />>/ />/  />/
>> />>/ />>/ /Well, I made a little script that did all the hard work of
>> checking
>> />>/ />>/ out a CVS version for specific date, configuring it with cmake,
>> />>/ />>/ building it, setting appropriate environment variables and then
>> />>/ />>/ running a little test application. The results indicate that
>> the
>> />>/ />>/ problems start with commits done on October 27. I suspect the
>> />>/ changes
>> />>/ />>/ relating to alpha handing (for bug 2347) in
>> />>/ vtkOpenGLRenderWindow.cxx
>> />>/ />>/ and friends, although I don't see anything suspicious in the
>> code.
>> />>/ />>/ Most of the changes seem related to depth peeling and using
>> />>/ different
>> />>/ />>/ blending, but I'm not enabling depth peeling and are also
>> not using
>> />>/ />>/ any transparent geometry.
>> />>/ />>/
>> />>/ />>/  />>/
>> />>/ />/ />/ Okay, further testing shows it is indeed related to using a
>> />>/ different
>> />>/ />/ blending function. If I take the faulty revision and only
>> comment out
>> />>/ />/ the calls to glBlendFuncSeparate in vtkOpenGLRenderer.cxx and
>> />>/ />/ vtkOpenGLRenderWindow.cxx the problem goes away, i.e.
>> performance is
>> />>/ />/ restored. So it looks like a software fallback is used as
>> />>/ side-effect of
>> />>/ />/ this call. Yuck. The card (actually driver) says it's OpenGL 1.5
>> />>/ />/ capable, and glBlendFuncSeparate was introduced in 1.4. But it
>> />>/ doesn't
>> />>/ />/ seem to be hw-accelerated...
>> />>/ />/ />/ I guess there's not much of a work-around for this,
>> except a card
>> />>/ />/ upgrade :-/
>> />>/ />/ />/ Paul
>> /
>>
>> -- 
>> François Bertel, PhD  | Kitware Inc. Suite 204
>> 1 (518) 371 3971 x113 | 28 Corporate Drive
>>                      | Clifton Park NY 12065, USA
>>
> 
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-- 
François Bertel, PhD  | Kitware Inc. Suite 204
1 (518) 371 3971 x113 | 28 Corporate Drive
                      | Clifton Park NY 12065, USA